1. Field of the Invention
The invention relates to a method and an apparatus for fabricating a semiconductor device, and more particularly to such a method and an apparatus for providing a flat insulative layer with a semiconductor device.
2. Description of the Related Art
With a need for higher integration of a semiconductor device, interconnections for communicating transistors, of which a semiconductor device is composed, with each other have to be thinner in diameter. Further with more complexity of a structure of a semiconductor device, such interconnections have to be formed in multi-layered structure. Such thinner diameter interconnections and multi-layered structure interconnections cause a spacing between interconnections to be in a sub-micron order. A conventional plasma-enhanced chemical vapor deposition, which is hereinafter referred to simply as "PE-CVD", is no longer able to bury such a quite narrow spacing with a silicon layer. The conventional PE-CVD cannot provide an appropriate step coverage of a silicon layer, and hence it is unavoidable that there remain large voids in a narrow space between interconnections. In subsequent semiconductor fabricating steps, humidity and/or impurities to be used in such steps tend to remain in the voids, thereby remarkably deteriorating performance and reliability of the products.
As a solution to such a problem, there has been suggested a silicon oxide layer to be produced by means of plasma-enhanced chemical vapor deposition with radio-frequency field being applied to a semiconductor substrate (hereinafter, this is called as bias plasma CVD). The silicon oxide layer produced by the bias plasma CVD is able to bury a quite narrow space therewith, however, is not able to protect active portions of a semiconductor device from humidity and/or impurities to be used in semiconductor device fabricating processes. Thus, there has been also suggested a silicon oxynitride layer to be produced by means of the bias plasma CVD in place of the above mentioned silicon oxide layer.
For instance, Japanese Unexamined Patent Public Disclosure No. 3-38038 has suggested a method for depositing an insulative layer on a substrate having a step by means of bias electron cyclotron resonance plasma-enhanced chemical vapor deposition. This method uses a silane family gas and N.sub.2 O as process gases to control either microwave outputs or a flow rate ratio of the silane family gas to N.sub.2 O, to thereby form an insulative layer which can bury narrow spacing between interconnections therewith. An experiment was actually carried out in the Disclosure on condition that N.sub.2 O and SiH.sub.4 gases were introduced at 35 sccm and 21 sccm, respectively, the operating pressure for forming the insulative layer was 7.times.10.sup.-4 Torr, the radio-frequency (RF) power was 500 W, and the microwave output was in the range of 200 to 1000 W. In another experiment, a flow rate ratio of SiH.sub.4 gas to N.sub.2 O gas was set to be 0.5, the operating pressure for forming the insulative layer was 7.times.10.sup.4 Torr, the radio-frequency (RF) power was 500 W, and the microwave output was 1000 W. The Disclosure describes that it is possible to form an insulative layer having an appropriate burning characteristic under the foregoing conditions.
For another instance, silicon nitride formation from a silane has been suggested by J. C. Barbour, H. J. Stein, O. A. Popov, M. Yoder and C. A. Outten in "Silicon nitride formation from a silane-nitrogen electron cyclotron resonance plasma", J. Vac. Sci. Technol. A, Vol. 9, No. 3, pp 480-484, May/June 1991. According to the article, low temperature silicon nitride and oxynitride films were deposited downstream from an electron cyclotron resonance plasma source using SiH.sub.4 and N.sub.2 gas mixtures. The Si/N ratio and H content in the deposited films were determined using Rutherford backscattering spectrometry and elastic recoil detection. The H concentration was minimum for films with compositions closest to that of stoichiometric Si.sub.3 N.sub.4. The optimum conditions for producing a stoichiometric Si.sub.3 N.sub.4 were a SiH.sub.4 /N.sub.2 flow ratio between 0.1 and 0.2, and an electrically isolated sample far from the ECR source. Infrared absorption spectra showed that as the film composition changed from N rich to Si rich the dominant bonds associated with H changed from N--H to Si--H.
The methods for fabricating a semiconductor device having a multilayered interconnection structure by using a conventional process for forming a silicon oxynitride layer has problems as follows. Firstly, the conventional methods cannot individually control amounts of nitrogen and oxygen to be present in a silicon oxynitride layer, and accordingly cannot provide a desired quality of the silicon oxynitride layer. Secondly, the performance of the conventionally formed silicon oxynitride layer to bury a space located between interconnections therewith is mainly dominated by sputtering effect of an argon gas and bias radio-frequency power, and thus less dominated by a gas flow rate of silane family gas and N.sub.2 O gas mixtures and microwave power. FIG. 1 illustrates the deposition of a conventional silicon oxynitride layer. On a substrate 1 are arranged metal interconnections 2 with a spacing S being given between the adjacent interconnections 2. When the spacing S is small, and further when a height H of the interconnections 2 is large, in other words, an aspect ratio of the interconnections 2 is large, it was not possible for the conventional silicon oxynitride layer 3 to avoid generation of voids 4 in the space located between the adjacent interconnections 2.
As earlier explained, the conventional processes for forming a silicon oxynitride layer cannot control the quality of the layer, and also cannot provide desired performance to bury a space located between adjacent interconnections therewith.